The present invention relates to a vaporizer for delivering vaporized reactants to a reaction zone, and to a method of using such vaporizer.
Glass particle deposition processes have been used to form preforms from which optical fibers can be drawn. In the glass particle deposition process, the vapor mixture is reacted at a burner or within a glass substrate tube to form a coating of glass particles which is subsequently fused to form a high quality glass. In order to enhance the fatigue resistance or other mechanical properties of the optical fiber, or to effect a change in the index of refraction of the vapor deposited optical fiber preform, the chemical composition of the vapors which are reacted to form the deposited glass particles may be varied. For the sake of simplicity, only burner-type deposition systems will be discussed. Typically SiCl.sub.4 or organosilioxane (octamethylcyclotetrasiloxane) is the primary vapor constituent. One or more additional vapors can be supplied to the oxidation/flame hydrolysis burner, the one or more vapors comprising chemical precursors of dopants whose presence affects the properties of the glass being formed.
In order to form a preform having generally consistent properties, and to assure an even distribution of the glass particles, it is necessary to supply the burner with a substantially constant flow of vaporized source material entrained in a carrier gas, such as O.sub.2, i.e. pressure spikes should be minimized. Moreover, the rate of change of flow of vapor should accurately track any change in vapor demand initiated by the control system. Accordingly, systems have been devised for controlling the carrier gas flow and the rate at which source material is vaporized and entrained into the carrier gas.
The reactant flow is typically measured in the vapor state. Alternative systems have been disclosed which meter the reactants in their liquid state, and thereafter vaporize or nebulize the reactants prior to their introduction into an oxidation/flame hydrolysis burner.
In one system wherein liquid reactants are metered in the liquid state, the metered reactants are supplied to a flash evaporation chamber. Oxygen is also supplied to the chamber and intermixed with the vaporized reactants prior to delivery to the vapor deposition means. Although the liquid reactants are delivered to the chamber in controlled amounts, the liquid is sprayed onto a heated surface whereby immediate vaporization occurs, thereby creating nucleate or film boiling. The vaporized output from this system slowly responds to the input liquid flow since the chamber volume needs to be large to permit the liquid to be sprayed onto or toward the heated plate. Thus, a step change in the flow of boron-containing reactant, for example, would not produce a step change in the amount of boron-containing particles deposited on the preform.
Another known reactant vaporizer employs a chamber including a cylindrical inner wall; reactants are fed to one end of the cylindrical chamber, and a thin film is formed by rotating blades. Since the blade shaft passes through an end wall of the chamber, the chamber is subject to leaking at the seal. Film thickness is essentially determined by the gap between the blades and the inner wall. It would appear that part of the film would not be thin since there would be a thick portion (like a wave) adjacent the stirrer until axial flow made the film thinner than the gap between the blade and inner wall. Liquid running down the blade could form droplets that could be carried in the vapor stream. Faster blade rotation would worsen the effect. Another disadvantage of this type of vaporizer is slow response time, the volume within the cylindrical chamber being relatively large.
Another cylindrical vaporizer includes a cylindrical heating rod that is situated within a cavity located within a chamber cylinder. The relative sizes of the inner heating rod and the surface that forms the cylindrical cavity is such that an annular gap exists between them. Liquid reactant flows onto the inner rod at one end of the vaporizer, the thickness of the reactant liquid film being mechanically constrained by the gap thickness. The temperature of the heating element is maintained below the temperature where nucleate or film boiling of the liquid occurs. It is difficult to increase flow in such a device. Vapor is generated at the leading edge of film, which is the only exposed surface. If flow of the liquid reactant were increased, and input power were increased to maintain a constant temperature, a point would be reached at which vapor would form at the surface of the inner cylindrical heater, i.e. liquid would flash off the surface, so that the vaporizer operation would become unstable. Vapor forming in the liquid behind the front edge would cause pressure spikes. Even when this type of vaporizer is operating well, pressure fluctuations of 2 to 3 mm Hg can occur. Moreover, this device is geometrically constrained to operate at very low flow rates.
It is therefore and object of the present invention to provide an improved system for delivering precise, controlled amounts of reactants at high flow rates to a glass particle deposition apparatus. Another object is to provide a vaporization chamber in which the flow of unvaporized liquid is undisturbed by vapor exiting the chamber. A further object is to provide a vaporization chamber that is simple in construction and has no moving parts. Yet another object is to provide a vaporizer the output vapor flow of which quickly and accurately tracks the input flow of reactant liquid.